In industry, unsaturated monomers, preferably methacrylic or acrylic acid esters, styrene and mixtures thereof, are mainly polymerized with initiators which decompose on heating into free radicals which subsequently react with a respective monomer molecule and trigger radical chain polymerization.
The initiators are azo compounds such as azodiisobutyrodinitrile (AIBN), peroxy esters and diacyl peroxides such as dibenzoyl peroxide. The polymerization temperatures depend on the decomposition constants of the initiators. Usually a temperature of at least 60 to 70.degree. C. is required. Since atmospheric oxygen reacts as a diradical with initiator radicals and growing polymer radicals and thus reduces the molecular weight of the polymers, polymerization using thermally activatable initiators must usually be carried out under oxygen-free conditions, e.g. in a protective gas atmosphere.
Homopolymerization and copolymerization of unsaturated monomers can be effected in bulk or in organic solvents or in aqueous emulsion or dispersion. Emulsion polymerization can also be carried out at 0 to 50.degree. C., preferably at room temperature, if "redox initiators" are used. Initiating radicals are formed preferably by reduction of peroxy compounds such as H.sub.2 O.sub.2, hydroperoxides or diacyl peroxides with oxidizing agents such as salts of transition metals, preferably salts of Fe.sup.2+, Co.sup.2+, Ti.sup.3+, tertiary amines such as N,N-dimethylaniline, dimethylaminotoluidine or N,N-dimethylbenzylamine, sulphites such as NaHSO.sub.3, sugars or ascorbic acid. Since the redox components are usually only soluble in water and radical formation also depends on the pH, their use is restricted mainly to aqueous systems.
Individual redox pairs such as Co.sup.2+ -carboxylate/hydroperoxide or diacyl peroxide/N,N-dimethylbenzylamine, are also suitable for crosslinking styrene solutions of unsaturated polymers containing fumarate and maleate groups at room temperature, and are used for example in the production of materials for jointless flooring. One disadvantage of these redox systems, however, is their sensitivity to oxygen, which increases the time taken by the surface to harden. This inhibition by oxygen can be partly prevented by addition of waxes, which float on the surface and thus form a barrier. Redox initiators of this kind can admittedly also be used for polymerization and crosslinking of acrylates and methacrylates, but in that case the polymerization or crosslinking are slower and less complete. Problems are caused both by inhibition by oxygen and by discoloration of the products. Also, individual redox components such as diacyl peroxides dissolve preferably in aromatic hydrocarbons such as styrene, and are difficult to dissolve in other monomers such as unsaturated esters.
Another possible method of room-temperature polymerization or crosslinking is photopolymerization. In this case the unsaturated monomers are mixed with radical initiators which have to be illuminated with UV before decomposing into polymerization-initiating radicals. Photopolymerization is also inhibited by oxygen.
Crosslinking of "alkyd" resins, made up of mono- or polyunsaturated fatty acids, polyols and cyclic anhydrides, is the only process which occurs in the presence of atmospheric oxygen even at room temperature. To this end, these lacquers are mixed with transition metal carboxylates, preferably Co.sup.2+, Zn.sup.2+, Mn.sup.2+, Pb.sup.2+ stearates or naphthenates, either singly or mixed in concentrations of 0.1 to 2 wt. %. Atmospheric oxygen causes hydroperoxides and peroxides to form on carbon atoms in the .alpha.-position relative to the double bonds of the unsaturated fatty-acid esters, and reacting in the presence of transition metal ions in a number of reactions involving crosslinking. A brief description of this so-called oxidative drying is given in Resins for Surface Coatings, Volume 1 (Ed. P. Oldring, G. Hayward), pages 52 ff (SITA Technology, London, 1987). However, this room-temperature crosslinking process in air is not a case of radical or ionic chain polymerization via double bonds.